The intermediary told Channel 4 News that the al-Qaeda-affiliated group, Boko Haram, has threatened to kill the remaining hostages if there is any attempt by the Nigerian military to rescue the girls. Meanwhile in the capital Abuja, the Nigerian government is coming under intense public pressure to secure the girls' release.

On Wednesday, the government admitted for the first time that it had officially engaged the services of the negotiator. It declined to reveal his identity. He has told Channel 4 News that he wishes to remain anonymous for reasons of personal security.

The intermediary has, however, been in regular, direct contact with senior members of the Islamist insurgent group who claim to be holding the girls, for several days now. He has maintained links with Boko Haram for nearly a decade and has successfully negotiated past hostage releases.

It is not clear how many of the 220 teenagers remain in Nigeria, following reports that they had been split into smaller groups and some moved into neighbouring Chad and Cameroon and sold for a bride-price of less than £10 each, following their forcible conversion to Islam.

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Boko Haram has not as yet claimed responsibility for the mass kidnap, which happened in the town of Chibok in north-eastern Borno State 17 days ago, but the group is considered the only likely perpetrator. The girls were snatched from their dormitories in the small hours of the morning of 15 April. The government secondary school was then set ablaze.

Earlier this week, following initial contact with the intermediary, the abductors had agreed to provide a list of the names of the schoolgirls they claim they're still holding, as proof of life. So far, they have not delivered this list.

The intermediary, who has relayed the abductors' demands to Kashim Shettima, Governor of Borno State, told Channel 4 News that Boko Haram is actively "considering a deal" by which most of the girls who remain in Nigeria could be released. The governor is currently in Abuja but could not be reached for comment.

The group did not explain to the intermediary what had happened to the three girls it says had died. It is possible that they were killed while attempting to escape, as 50 of the girls have now done. Reports in other media cite a community elder in Chibok as saying that he too had learned of some deaths, reportedly from snakebites.

But those familiar with the brutal practices of Boko Haram say it is just as likely that the girls had been subjected to extreme sexual violence. This is something of which the girls' parents as well as women's groups have expressed grave fears.

Drug-taking among members of the group is commonplace and they are often high on drugs when they launch frenzied attacks. The intermediary told Channel 4 News that the longer the crisis was allowed to drag on, the worse - and more complex - the situation would get.

Hundreds of Nigerian women yesterday marched through the capital, Abuja, demanding urgent government action to secure their release.

This morning, more than 20 members of the Nigerian Senate met President Goodluck Jonathan to discuss the abductions. They demanded daily information briefings from the government and the military to inform the nation as to what was being done to tackle the crisis.

In past crises, members of the Islamist insurgent group, Boko Haram, have executed hostages when the army has launched an attack or rescue-bid and there are fears that with pressure mounting for fast results, military intervention could have tragic results.

'Western education is forbidden'

Boko Haram has, however, released hostages following negotiations. Most of these have involved the intermediary, whom Channel 4 News has been in contact with since Monday.

Boko Haram claims to be fighting for a strict Sharia state across northern Nigeria. But the guiding influence of Salafist imams has demonstrably waned as the group has fractured and hijacked as an al-Qaeda franchise, with tendrils throughout northwest Africa's Islamic Maghreb.

Many atrocities - from deadly bomb attacks on soft targets, to assaults on schools, churches and even mosques - are claimed by the Taliban-style group, whose name in the Hausa language translates as "western education is forbidden."